Colorado potato beetle (CPB;Leptinotarsa decemlineata Say,Coleoptera:Chrysomelidae) has shown a remarkable adaptability to a variety of control measures.Although oryzacystatin Ⅰ and Ⅱ (OCⅠ and OCⅡ) have potential in controlling pests that use cysteine proteinases for food digestion,expression of a single OC gene in potato exhibited a minimal or no effect on CPB fitness traits.The aim of this study was to examine the effect of coexpressed OCⅠ and OCⅡ in potato (Solanum tuberosum L.) cultivars Desiree,Draga(c)evka and Jelica on CPB larvae.Growth parameters,consumption rates and food utilization,as well as activity of proteases of CPB larvae were assayed.Second and third instar larvae fed on transformed leaves molted earlier and had higher relative growth and consumption rates than larvae fed on nontransformed leaves,while efficiency of food utilization was unaffected.In contrast,fourth instar maximum weight gain and amount of leaves consumed were about 20％ lower for the larvae fed on transgenic potato.Analysis of total protease activity of third instar larvae revealed reduction in overall proteolytic activity measured by azocasein hydrolysis,accompanied with inhibition ofcysteine proteinase activity 24 h after ingestion of potato leaves expressing OCⅠ and OCⅡ.However,after long-term feeding on transformed leaves proteolytic activities of larvae became similar to the controls.Although feeding on OCⅠ/OCⅡ leaves did not affect larval survival,coexpression of OC genes reduced the development time and thus significantly decreased plant damage caused by CPB larvae.